Steel that is to be fabricated into articles such as food cans, beer and beverage cans, etc. is normally subjected to a number of pretreatment steps prior to shipment to the producer of the article. In a typical pretreatment line, a continuous coil of steel is covered by a layer of electrolytically deposited tin which may have a small amount of tin oxide on its surface, over which is deposited a discontinuous layer of chromium and/or chromium oxide for example, by cathodic treatment in a sodium dichromate solution, after which it is subjected to a water rinse, after which a generally thin layer of lubricant is applied, after which it is dried and recoiled for shipment.
As a result of the tinplate and chromium treatment steps, the surface of the electrolytic tinplate has areas which are rich in tin and its oxides. Such material is prepared at the steel mill and can be ordered with varying weights of steel plate by the can makers convention of so many pounds of steel per base box which equals 31,360 square inches of plate with 62,720 square inches of surface (both sides). The amount of electrolytically deposited tin is designated by a fraction of a pound per base box (62,720 square inches).
Bare tinplate surfaces usually contain electrolytically deposited chromium or its oxides which passivates the tin surface to prevent tin oxide growth in warm humid environments. In the past such oiled metal surfaces were frequently flame or corona treated to reduce or oxidize these organics; however, these techniques are slow and energy intensive and require capital investment for expensive line equipment and space in the plant. The chromium, while serving the useful function of passivating the surface, is nevertheless a malefactor that interferes with adhesion of subsequently applied coatings and or inks.
In order to provide surface mobility to prevent abrasion during shipping or handling of the tinplated steel, the steel mills usually deposit electrostatically onto the tinplate surface a very thin coating of lubricant such as acetyl tributyl citrate, dioctyl sebacate or the like. Thus, the material as received has a lightly oiled metallic surface having areas of tin and its oxide and areas of chrome and its oxides. The commonly used lubricants are acetyl tributyl citrate, dioctyl sebacate or butyl stearate, all of which are relatively nonoxidizing liquids. When they are applied to the metals at a rate of 0.17-0.28 grams per 62,720 square inches of surface (per base box), a coating is formed slightly in excess of a monomolecular layer which acts to increase mobility and prevent abrasion during handling and shipment.
Electrolytic tinplate has been difficult to cover with adhesive resin systems. It has therefore been necessary to prime the surfaces of coil with a substance capable of adding reactive sites for anchoring to the base substrate and overcoating.
Organic titanates are known to be useful for promotion of adhesion of inks and coatings to metal surfaces and have been used as primers and/or to modify inks and coatings for improved adhesion to the metal substrate.
Application of a titanate ester primer to cover the lubricants, contaminants, chromium and its oxides followed by subsequent treatment to activate the titanate, such as treatment with ultraviolet light, transforms the surface permitting subsequent application of ultraviolet or thermally cured inks or coatings. The bare surface of tinplate including the areas rich in chromium oxides and others of pure tin oxides become coated with an attached layer of hydrated titanium dioxide gel and thereby provides a satisfactory adhesive base for subsequent inks or coatings.
In my U.S. Pat. No. 4,228,221 dated Oct. 14, 1980, entitled "Primer System", there is disclosed a system wherein a titanate ester and ultraviolet treatment are employed to condition the surface of an organic lubricant over electrolytic tinplate such that subsequently applied ultraviolet activated inks could be firmly adhered. According to this procedure, organic titanates on metal surfaces are subjected to ultraviolet treatment to increase adhesion of inks and coatings. The method includes an initial treatment of lubricated metal with an organic titanate such as tetrakis-(2-ethyl hexyl) titanate and then exposing the treated metal to ultraviolet light to accelerate the reaction of the titanate with the metal surface. In this procedure, titanate esters or chelates are applied to metal surfaces including lubricated surfaces, such as electrolytic timplate, tin free steel, tin, stainless steel, chrome plated steel, galvanized steel and aluminum, by application from solvent mixtures or by direct contact after which the metal surface is subjected to ultraviolet light activation for oxidative polymerization of nondrying oils and for accelerated hydrolysis of titanate esters. Activation by ultraviolet radiation of organic titanate over lubricated metallic surfaces prior to printing or coating was found to prepare such surfaces for reception of a subsequently applied ink or coating. The application of ultraviolet energy subsequent to priming with an organic titanate was found to improve the bonding of the titanate to the substrate and the bonding of inks and coatings subsequently applied thereover.
While the process and primer of the aforesaid patent are highly satisfactory, they largely contemplate the use of solvent-based titanates, rely on ultraviolet light for activation and may lead to contamination of oiling or lubricating equipment with titanate.
The present invention is an improvement of the invention described and claimed in said patent in which environmentally detrimental organic solvents are eliminated and contamination of equipment with titanate is minimized. While ultraviolet light may be employed herein, if desired, it is not essential for activation of the system employed thereby rendering the system more versatile.